"中国的计划生育策略是一个相信科学的重大失误。"

This is something I couldn't understand. The math seems simple: in a simplified situation, if each couple makes two children, that is, every two people making two people to fill in their share in the population once they die, the population will stay constant.

That also means, the young population halfs if each couple makes only one child. Two generations later, the young population will be 1/4 of their grandparents' generation. Therefore, the one-child policy is obviously wrong and even dangerous!

The authority then must have been smart enough to see this, then why did they still implement it? Am I missing something, at least mathematically?